The terminology “steel for packaging” includes all steel-based materials used for the packaging of food products, beverages and industrial non-food products such as aerosol gases or paint.
Packaging steels include, among other things, tin plate, which is steel having low carbon content, generally less than 0.08%, covered by a layer of tin.
A passivation layer is applied after the deposition of the tin, and makes it possible, among other things, to limit the development of tin oxides. The passivation can be accomplished by depositing an aqueous passivation solution such as a phosphating or chrome plating solution or a solution of soluble compounds such as titanium oxalate, titanium sulfate or zirconium sulfate. The deposit is first applied in the form of a wet film, wherein the solution is in aqueous form, and then the coated product is dried to obtain a dry film. The drying is generally performed by passing the coated strip through a tunnel equipped with nozzles that spray hot air in the direction of the strip. The thickness of the wet film deposited is generally less than 5 μm, for a dry passivation coating thickness less than 10 nm. Indeed, the passivation coating is mechanically weak and the greater its thickness, the higher the risk of cohesive fracture. This cohesive fracture results in delamination or loss of adherence of organic coatings, such as varnishes or paints, which may be applied later.
The passivation layer may be deposited by a “spray/dip/squeeze” method, according to which the surface to be coated is placed in contact with the aqueous passivation solution by spraying or by immersion, after which the excess solution is eliminated with the aid of a squeeze roller. The disadvantage of this method is that the thickness of the wet film deposited is not a function of the quantity of solution placed in contact with the strip but on the speed of the coating line. The quantity of solution removed with the aid of the squeezing rollers is notably a function of the speed at which the strip travels between these rollers. Consequently, with this method it is difficult to obtain a uniform thickness of the passivation layer, regardless of the speed of the line.
Another method for the deposition of a passivation layer includes spraying the aqueous passivation solution on the strip in the form of a uniform flow of droplets generated by rotating discs, and then smoothing out the passivation layer with the aid of a spreader roller. This method has the advantage that the thickness of the wet film deposited can be kept constant even in the event of a variation in the speed of the line by adjusting the flow of solution sprayed by the rotating discs. However, the uniform spreading of the solution depends to a great extent on the time necessary for the drops to form a uniform liquid stream and on the contact force between the spreader roller and the strip, which is not easy to control due to the wear of the rollers, variations in the flatness of the strip and tolerances in the geometry and alignment of the rollers. Therefore, there are defects in the uniformity of the thickness of the passivation layer that are unacceptable for packaging applications.
To eliminate these problems of uniformity it is possible to increase the quantity of solution sprayed by the rotating discs, but it leads to a higher average thickness of the wet film which in turn causes drying problems. Indeed, an increased quantity of water must be evaporated to achieve the required dry thickness of the passivation layer and to eliminate the presence of residual moisture on the strip. One possibility is then to increase the time it takes the strip to pass through the drying installation by slowing down the line, which poses productivity problems, in particular for this type of manufacturing line which generally operates at a speed greater than or equal to 400 m/min.
Another solution could be to increase the length of the drying unit, but that causes space problems on the line. Another possibility includes increasing the temperature of the air blown toward the strip, although this increase results in an increase in the energy consumption and a deterioration of the passivation solution.